I'd doubt you'll find the kit for much less than that. The wholesale cost on those Bose 901 surrounds is probably over a buck each, so double that (standard margin on "accessory" type items for 99% of all stores), and you get $36.
Hard to get very cheap, when you're talking about EIGHTEEN items... even at $36, that's only $2 each... cheaper than MAT for single surrounds!
Regards,
Gordon.